The present invention relates to a galvanized steel sheet, a method for manufacturing the same, and a method for press-formed product.
Demand of galvanized steel sheets having superior rust-preventive performance increases as thin steel sheets for automobiles, household electric appliances, and building materials. Galvanized steel sheets used for press-forming are required to have an adequate level of surface roughness, or of microscopic roughness profile on the surface thereof because the microscopic roughness increases the retainability of lubrication oil between the work (galvanized steel sheet) and a press-mold, decreases the sliding resistance of the work, and prevents the occurrence of die-galling.
Generally a mean roughness Ra defined in JIS B0601 is adopted as an index of the texture of microscopic roughness on the surface of steel sheet. For galvanized steel sheets used in press-forming, generally the oil-retainability between the work and the mold during the press-forming is assured by regulating the mean roughness Ra within a specified range.
Other indexes, however, such as the maximum height of roughness profile, Rmax, and the ten-point height of roughness profile may also be applied. Alternatively, JP-A-7-136701, (the term xe2x80x9cJP-Axe2x80x9d referred herein signifies the xe2x80x9cUnexamined Japanese patent publicationxe2x80x9d), defines the sum of the volumes of profile valley portions per unit are as the index, and gives evaluation of excellent press-formability for the index larger than a specified value thereof. In any case, the press-formability cannot be assured unless the surface of target galvanized steel sheet has a certain level of microscopic roughness profile.
Particularly for galvanized steel sheets that have a coating film consisting mainly of xcex7 phase, the film is soft and has low melting point compared with the surface of alloyed hot-dip galvanized steel sheets, so they likely induce adhesion to the press-mold and may degrade the press-formability. Consequently, that type of galvanized steel sheets has to assure increased oil-retainability. With these reasons, that type of galvanized steel sheets are often requested to have relatively large values of height of roughness profile on the surface, or mean roughness Ra, necessary to assure the press-formability compared with alloyed hot-dip galvanized steel sheets.
On the other hand, the galvanized steel sheets used in exterior plates of automobiles and the like are requested to have press-formability and also excellent image clarity after painting. Therefore, to improve only the image clarity after coating, the surface of galvanized steel sheet is finished to a bright face. However, improvement in the press-formability needs to establish a certain level of surface roughness. The two requirements conflict with each other.
The relation between the image sharpness after painting and the microscopic texture of surface of steel sheet is described in, for example, JP-B-6-75728, (the term xe2x80x9cJP-Bxe2x80x9d referred herein signifies the xe2x80x9cExamined Japanese patent publicationxe2x80x9d). According to the disclosure, since the coating film itself acts as a low pass filter to the microscopic roughness profile on the surface of steel sheet, the short-period roughness profile is covered by the coating film, thus the short-period roughness profile does not give influence on the image sharpness after coating. On the other hand, the long-period roughness profile portions having wavelengths of several hundreds of micrometers or larger are not covered by the coating, thus degrading the image sharpness.
A countermeasures to the phenomenon is to regulate the filtered centerline waviness Wca which is an index for expressing the microscopic roughness profile on the surface of steel sheet before coating to not exceed a certain level, thus improving the image sharpness after coating. The term xe2x80x9cfiltered centerline waviness Wcaxe2x80x9d is a parameter that is defined by JIS B0610, and represents the mean height of roughness profile on the surface after treated by high-pass cut-off.
Other than the filtered centerline waviness Wca, peak count PPI is applied as an index of influence on the image sharpness after coating. As specified in SAE 911 Standard, the peak count PPI is the number of peaks of roughness profile per one inch length. Large peak count means the large number of short-period roughness profile in the microscopic roughness profile on the surface, or, when compared on the same mean roughness Ra, the long-period wave length components are relatively decreased. That is, if the mean roughness Ra is the same, larger peak count PPI should give superior image sharpness after coating.
Consequently, the galvanized steel sheets for press-forming use need to have a surface roughness with a certain level of microscopic roughness profile, and, when the image sharpness after coating is required, the long-period components are necessary to be decreased. In particular, different from the alloyed hot-dip galvanized steel sheets that form microscopic roughness profile on the surface thereof during alloying stage, the galvanized steel sheets that have a coating film consisting mainly of xcex7 phase give smooth surface thereof after coating, so there is a strong need of giving surface roughness by some means.
Temper rolling is applied as a means to give microscopic roughness profile on the surface of galvanized steel sheets used for press-forming. The temper rolling is a means that uses a rolling roll having microscopic roughness profile on the surface thereof, and that applies a plastic extension in an approximate range of from 0.5 to 2.0% to the steel sheet, thus inducing a pressure on the roll byte to transfer the roughness profile on the surface of the rolling roll to the surface of the steel sheet. Therefore, the texture of microscopic roughness profile formed on the surface of galvanized steel sheet depends on the texture of roughness profile on the surface of the rolling roll.
The applicable method to form microscopic roughness profile on the surface of temper rolling roll includes shot-blasting, electrical discharge machining, laser beam machining, and electron beam machining. For example, JP-A-7-136701 and JP-B-6-75728 disclose a method that uses a temper rolling roll finished by laser dull treatment, and JP-A-11-302816 discloses a method that uses a temper rolling roll finished on the surface thereof by electron beam machining.
Zinmnik et al. (Stahl und Eisen, Vol.118, No.3, pp.75-80, 1998) reports a method to increase the peak count PPI on the surface of steel sheet using a temper rolling roll, which method is called the xe2x80x9cPretex processxe2x80x9d. According to the report, hard metallic chromium is electrically deposited to form microscopic roughness profile on the surface of rolling roll. Zinmnik et al. describe that the method can create short pitch and dense roughness profile compared with the rolling-roll surface machining by shot-blasting.
According to the report, a rolling roll with shot-blast finish creates around 120 of peak count PPI on the surface of steel sheet, and the Pretex process can increase the peak count PPI to around 230. The threshold of the peak count PPI given in the report is xc2x10.5 xcexcm, (in contrast, the threshold of the peak count PPI referred in the descriptions is xc2x10.635 xcexcm).
The related art applying temper rolling, which is used as a method to provide a certain level of surface roughness on the surface of galvanized steel sheet for press-forming, has problems described below.
First, the degree of transferring the microscopic roughness profile on the surface of a rolling roll by the temper rolling onto the surface of a galvanized steel sheet has a limitation. Thus, even when the surface of rolling roll has fine roughness profile, all of the profile cannot be transferred onto the surface of steel sheet, and the peak count PPI on the surface of the steel sheet cannot be increased.
The temper rolling transfers the microscopic roughness profile on the surface of rolling roll onto the surface of steel sheet while applying a certain level of plastic extension onto the steel sheet utilizing the pressure induced in the roll byte. The main function of the temper rolling is, however, to adjust the mechanical properties of the steel sheet after annealing, so the maximum extension to achieve the objective has a limitation. Therefore, to almost completely transfer the microscopic roughness profile on the surface of rolling roll onto the surface of steel sheet, the pressure induced in the roll byte may be significantly increased. In that case, however, the bulk deformation of the steel sheet becomes excessive, and the mechanical properties of the steel sheet degrade.
For instance, with an objective of adjusting the mechanical properties of a steel sheet, when the extension that can be given by the temper rolling is in a range of from 0.5 to 2.0%, the mean roughness Ra on the surface of rolling roll is necessary be regulated to a range of approximately from 2.5 to 3.5 xcexcm to obtain the mean roughness Ra on the surface of steel sheet in a range of from 1.0 to 1.5 xcexcm. In that case, to increase the peak count PPI on the surface of the rolling roll, even when the rolling roll is machined using electrical discharge machining, electron beam machining, or the like, the attainable peak count PPI on the surface of the rolling roll is around 300 at the maximum. Since the degree of transfer of the peak count PPI by the temper rolling in that case is in an approximate range of from 60 to 70%, the peak count PPI of the microscopic roughness profile transferred onto the surface of steel sheet is around 200 at the maximum.
For example, the above-referred JP-A-11-302816 discloses a technology of applying the electron beam machining onto the surface of rolling roll. The embodiments of the disclosure describe the pitch of peaks and valleys of surface roughness profile of a galvanized steel sheet of about 0.11 mm, which suggests the number of peaks and valleys of roughness profile per one inch length of about 230. The above-described Pretex method also provides around 230 of the peak count PPI on the surface of steel sheet. Thus, the existing technologies cannot give further dense short-wave length roughness profile on the surface of steel sheet.
Particularly the galvanized steel sheets with a coating film consisting mainly of xcex7 phase often increase the mean roughness Ra compared with the alloyed hot-dip galvanized steel sheets, so the mean roughness to be given on the surface of the rolling roll has to be increased responding thereto. Since, however, the above-described various types of roll-surface finishing methods decrease the peak count PPI in the case of increasing the mean roughness on the surface of rolling roll, which results in difficulty in increasing both the mean roughness Ra and the peak count PPI at a time.
In the case that that type of galvanized steel sheets are used for press-forming, the oil retainability between the steel sheet and the press-mold is not sufficient, and the sliding resistance between them increases, thus inducing problems of likely generating break of the steel sheet at punch face or break of the steel sheet near the mold bead portion.
Secondly, a roll byte applied in the temper rolling gives very high contact pressure between the rolling roll and the steel sheet, so the microscopic roughness profile on the surface (surface roughness) of the rolling roll varies with time owing to wear, thus the texture of microscopic roughness profile being transferred onto the surface of steel sheet cannot be kept uniformly.
For example, when a rolling roll having a surface mean roughness Ra of 3.5 xcexcm is used, the temper rolling of about 6 km in rolling length degrades the mean roughness Ra on the surface of rolling roll to around 3.0 xcexcm. As a result, the mean roughness Ra on the surface of galvanized steel sheet also decreases from 1.5 xcexcm to around 1.3 xcexcm. The influence of the wear of the surface of rolling roll becomes significant with the increase in the rolling length. The resulting variations in the microscopic roughness texture on the surface of individual products induce differences in press-formability, which raises a problem of unstable quality.
Therefore, for keeping stable press-formability of steel sheets, the rolling roll is necessary to be replaced before the wear on the surface thereof significantly proceeds. The frequent replacement of the rolling roll degrades the production efficiency.
For the case of galvanized steel sheets having a coating film consisting mainly of xcex7 phase, larger Ra is often requested than the Ra of alloyed hot-dip galvanized steel sheets. Consequently, the surface mean roughness Ra on the rolling roll has to be larger, and the influence of the wear on the surface of rolling roll with time becomes significant. Furthermore, adding to the wear, the apparent surface roughness of the roll is decreased by adhering zinc powder separated from the steel sheet to the profile valley portions of the microscopic roughness profile on the surface of rolling roll (what is called the clogging), thus inducing variations in microscopic roughness profile on the surface of the producing galvanized steel sheets with time.
Thirdly, with the manufacturing methods of galvanized steel sheets in related art, when the grades or the like of the treating galvanized steel sheets change to vary the hardness of substrate sheets, it is difficult to attain consistent level of surface roughness.
The problem is described referring to FIG. 36. FIG. 36 shows a result of temper rolling on galvanized steel sheets using a rolling roll which was finished in the surface thereof to give a mean roughness Ra of 3.0 xcexcm by the electrical discharge machining.
The temper rolling was applied to a hard material (high strength steel of the substrate sheet) and to a soft material (soft very low carbon steel), both were treated by hot-dip galvanizing in advance. During the temper rolling, the applied extension was varied stepwise, and the mean roughness on the surface of each of the galvanized steel sheets was determined. The figure shows that the hard material gives larger mean roughness on the surface of the galvanized steel sheet resulting from the temper rolling than that of the hard material. The reason of the difference is that the contact pressure between the rolling roll and the steel sheet, generated during attaining a specified extension, increases more in hard material than in soft material, and higher contact pressure more likely induces the deformation of the galvanized film layer, which results in easy for transferring the microscopic roughness profile on the surface of rolling roll.
In some cases, both the hard material and the soft material have to regulate the surface mean roughness Ra to a range of from 1.0 to 1.2 xcexcm, and the extension during the temper rolling to a range of from 0.8 to 1.0% for adjusting the mechanical properties in view of assuring the press-formability of the steel sheet. In that case, the result given in FIG. 36 suggests that the soft material can manufacture a galvanized steel sheet that satisfies the requirements, and, however, the hard material fails in attaining the objectives even when the same rolling roll as that used for the soft material is applied.
Consequently, for the case of applying temper rolling to a soft material, the surface mean roughness Ra of the rolling roll has to be decreased below the above-given 3.0 xcexcm. That is, the objectives cannot be attained unless the rolling roll is replaced. In other words, within a range of extension limited by the target material, the same rolling roll cannot give the same surface roughness on galvanized steel sheets with different steel grades.
It is an object of the present invention to provide a galvanized steel sheet having excellent press-formability and a method for manufacturing the same.
To attain the object, the present invention provides a method for manufacturing a galvanized steel sheet, in which solid particles are blasted against the surface of the galvanized steel sheet to adjust the surface texture thereof.
The surface texture is preferably defined by at least one parameter selected from the group consisting of mean roughness Ra on the surface of steel sheet, peak count PPI on the surface of the steel sheet, and filtered centerline waviness Wca on the surface of the steel sheet.
The mean roughness Ra on the surface of steel sheet, the peak count PPI on the surface of steel sheet, and the filtered centerline waviness Wca on the surface of the steel sheet are preferably adjusted in a range given below.
(a) The mean roughness on the surface of steel sheet, Ra: 0.3 to 3 xcexcm
(b) The peak count on the surface of steel sheet, PPI: 250 or more
(c) The filtered centerline waviness on the surface of steel sheet, Wca: 0.8 xcexcm or less
The solid particles blasted against the surface of galvanized steel sheet preferably have mean particle sizes of from 10 to 300 xcexcm. The solid particles are preferably a metallic material. The solid particles are preferably in near-spherical shape.
The step of adjusting the surface texture is preferably conducted by blasting solid particles against the surface of galvanized steel sheet at blasting speeds of from 30 to 300 m/sec, thus adjusting the surface texture of the steel sheet. It is preferable that the solid particles are blasted against the surface of galvanized steel sheet at blasting densities of from 0.2 to 40 kg/m2, thus adjusting the surface texture of the steel sheet. Prior to the step for adjusting the surface texture, a step for temper rolling may be applied to adjust the centerline waviness Wca on the surface of galvanized steel sheet to 0.7 xcexcm or less.
The adjustment of the surface texture is preferably done using a wheel blast machine. The distance between the center of rotating wheel and the steel strip is preferably 700 mm or less. The solid particles blasted against the surface of galvanized steel sheet preferably have mean particle sizes of from 30 to 300 xcexcm.
When the mean particle size is signified by d, the solid particles preferably have 85% or higher weight percentage of the solid particles having sizes of from 0.5d to 2d to the total weight of the solid particles. The solid particles preferably have the densities of 2 g/cm3 or more.
Furthermore, the present invention provides a galvanized steel sheet having a surface of dimple-pattern texture.
The term xe2x80x9cdimple-patternxe2x80x9d referred herein designates a texture where the profile valley portions on the surface consist mainly of curved surfaces, or, for example, the texture having large number of crater-shape concavities formed by colliding near-spherical shape objects against the surface. With the large number of dimple-shape concavities, the concavities play a role of pockets for oil of press-forming, thus improving the oil-retainability between the mold and the steel sheet.
The surface preferably has mean roughness Ra in a range of from 0.3 to 3 xcexcm. The term xe2x80x9cmean roughnessxe2x80x9d referred herein designates the xe2x80x9ccenterline mean roughnessxe2x80x9d specified in JIS B0601.
The surface preferably has peak count PPI expressed by the formula:
xe2x88x9250xc3x97Ra(xcexcm)+300 less than PPI less than 600 
The term xe2x80x9cpeak countxe2x80x9d referred herein designates the number of peak portions of roughness profile on the surface per one inch length. The above-given peak count PPI is expressed by a value at xc2x10.635 xcexcm of the threshold.
The surface preferably has at least 250 of peak count PPI.
The surface has filtered centerline waviness Wca of 0.8 xcexcm or less. The term xe2x80x9cfiltered centerline wavinessxe2x80x9d referred herein designates the xe2x80x9ccenterline wavinessxe2x80x9d specified in JIS B0610, and represents the mean height of roughness profile after treated by high-pass cutoff.
The galvanized steel sheet preferably has a coating film consisting essentially of xcex7 phase.
The galvanized steel sheet preferably has number densities of concavities of 3.1xc3x97102 counts/mm2 or more at a depth level corresponding to 80% bearing area ratio.
The surface of the galvanized steel sheet preferably has a surface texture giving core fluid retaining indexes Sci of 1.2 or more.
The galvanized steel sheet preferably further has a solid lubrication film having thicknesses of from 0.001 to 2 xcexcm on the surface thereof. The solid lubrication film is preferably a film selected from the group consisting of an inorganic solid lubrication film, an organic solid lubrication film, and an organic-inorganic composite solid lubrication film.
The solid lubrication film is preferably a phosphorous-base oxide film prepared by applying and drying an aqueous solution containing phosphoric acid and at least one cationic component selected from the group consisting of Fe, Al, Mn, Ni, and NH4+.
The above-described solid lubrication film is more preferably the one described below.
(1) The solid lubrication film contains a P component and an N component, and at least one component selected from the group consisting of Fe, Al, Mn, and Ni. The solid lubricating film has a molar ratio (a)/(b) in a range of from 0.2 to 6, where (a) designates the amount of P component, and (b) designates the total amount of N component, Fe, Al, Mn, and Ni; the amount of P component is expressed by the P2O5 converted value, and the amount of N component is expressed by the ammonium converted value.
(2) The solid lubrication film contains the P component and the N component as the solid lubrication film components in a form of chemical compound selected from the group consisting of a nitrogen compound, a phosphorus-base compound, and a nitrogen-phosphorus-base compound.
(3) The solid lubrication film contains at least Fe as the solid lubrication film component.
The galvanized steel sheet having the above-described solid lubrication film is manufactured by the steps of: applying an aqueous solution containing a cationic component (xcex1) and a phosphoric acid component (xcex2) on the surface of plated layer of a galvanized steel sheet; and drying the applied film without washing thereof with water, thus forming a coating film. The cationic component (xcex1) consists substantially of at least one metallic ion or cation selected from the group consisting of Mg, Al, Ca, Ti, Fe, Co, Ni, Cu, Mo, and NH4+. The aqueous solution has a molar concentration ratio (xcex1)/(xcex2) ranging from 0.2 to 6, where (xcex1) designates the total amount of cations, and (xcex2) designates the amount of phosphoric acid component. The phosphoric acid is expressed by the value converted to P2O5 molar concentration.
Furthermore, the present invention provides a method for manufacturing press-formed product, which contains the first step of preparing a galvanized steel sheet member having a surface in dimple-pattern texture, and the second step of applying press-forming to the member to obtain designed shape of press-formed product.